The long term goal of our studies is to elucidate, employing systematic physiologic approaches, the neuroendocrine mechanisms that govern testicular function in the adult rhesus monkey, a representative higher primate. In the present proposal we intend to determine whether the testosterone induced slowing of the frequency of intermittent hypothalamic GnRH secretion is the major mechanism mediating the negative feedback control of gonadotropin secretion by the testis, to determine whether the hypothalamus of the female is endowed with the capacity to respond to the foregoing action of testosterone, to begin to examine the neural mechanisms underlying the testosterone induced retardation of intermittent GnRH secretion, and to evaluate the significance of the episodic nature of testicular testosterone secretion in the negative feedback control of gonadotropin secretion. Remote blood sampling procedures that do not require the use of anesthetics or restraint will be used to collect sequential plasma samples. Hormone concentrations in body fluids will be determined primarily by established radioimmunioassay procedures, and pulsatile patterns of LH, FSH and testosterone secretion will be analyzed by a computer executed algorithm. Hypothalamic lesions and implantations will be conducted using aseptic stereotaxic procedures previously described. These studies will provide important insights into the neuroendocrine control system governing testicular function in higher primates, and may therefore have direct relevance to fertility control and to the treatment of infertility in men.